Method and device for the generation of far ultraviolet or soft x-ray radiation

ABSTRACT

In a method for generating extreme ultraviolet radiation or soft x-ray radiation by means of gas discharge, in particular, for EUV lithography, a discharge vessel is provided with two electrodes that are connected to high voltage. Between the electrodes, in an area of two electrode recesses that are coaxial to one another, a gas fill with predetermined gas pressure in accordance with a discharge operation realized on the left branch of the Paschen curve is provided. In this area, a plasma emitting the radiation is generated when supplying energy. The plasma is displaced or deformed by a pressure change of the gas fill in the area of the electrode recesses.

The invention relates to a method for generating extremeultraviolet/soft x-ray radiation having the features according to thepreamble of claim 1.

Preferred fields of application for the extreme ultraviolet (EUV)radiation or the soft x-ray radiation in the range of approximately 1 nmto 20 nm wavelength are particularly the EUV lithography. WO 99/29145discloses a method having the features of the aforementioned kind. Thedevice used in this method is comprised of an anode with a central borerecess and an oppositely positioned hollow cathode. The device operatesin an environment of constant gas pressure. For generating EUVradiation, gases of the elements of the atomic number Z>3 are preferred,for example, Xe with broadband emission characteristics. When highvoltage is employed, gas firing occurs depending on the pressure and theelectrode distance. The pressure of the gas and the electrode distanceare selected such that the system operates on the left branch of thePaschen curve and, as a result of this, no dielectric firing between theelectrodes occurs. Only in the vicinity of the hollow cathode the fieldlines are sufficiently extended so that the firing conditions arefulfilled above a certain voltage. A current-conducting plasma channelof axially symmetrical shape is then formed between the electrodes inaccordance with the electrode recess. The electrical circuit connectedto the device is configured such that a very high discharge currentoccurs when the current-conducting channel is generated. This currentgenerates a magnetic field about the current path. The resulting Lorentzforce constricts the plasma. It has been known for a long time that thisconstriction effect can cause the plasma to be heated to very hightemperatures and can generate radiation of very short wavelength. It hasbeen demonstrated that the device can generate EUV light (10 to 20 nm)very effectively, that it enables high repetition frequencies, and has amoderate electrode wear.

The plasma emitting in the short-wave range is produced along an axis ofsymmetry in the area of the hollow cathode and past the recess of theanode, depending on the provided conditions. Relevant parameters for ageometry of the plasma are derived from the shape of the electrodes, aswell as parameters of a supplied electrical current, its duration, itsshape, and its amplitude, as well as the gas pressure conditions and thecomposition of the gas of the gas fill in the discharge vessel or in thearea of the electrodes.

The known method results in a pinch, i.e., in a plasma channel thatshould be shorter and whose radiation should be able to be decoupledfrom the electrode system in a better way.

Accordingly, it is an object of the invention to improve a method havingthe aforementioned features such that the decoupling action of theradiation from the electrodes is improved and in that an optimizedplasma geometry results, i.e., an emission area that is shorter axially.

The aforementioned object is solved by the features of thecharacterizing portion of claim 1.

It is important for the invention that a pressure gradient of the gasfill is used in order to displace and/or deform the pinch or the plasma.These measures result in an improvement of the decoupling action of theradiation from the electrodes, for example, in a focusing device of anEUV lithography station. The displacement of the plasma can be carriedout such that it is optically excellently accessible, i.e., shadingoccurs as little as possible, even when observing at large observationangles, relative to the axis of symmetry. Also, an optimal adaptation ofthe light throughput of the radiation source, i.e., of the plasma, tothe optical system can be obtained. The light throughput is determinedby the product of the effective surface of the plasma and the flareangle. In principle, a light throughput as minimal as possible isdesired, i.e., a point source, so that utilization of large proportionsof the light emitted into the half-space is ensured. In the context ofthe source geometry of an electrode discharge that is present here, theproblem is reduced substantially to a plasma that is emitting axially asshort as possible and for which essentially no radiation shading lossesshould occur.

The method can be described in more detail in connection with anadaptation of an EUV light source to an optical device of an EUVlithography station. The EUV light source can be used in a semiconductorlithography device of the next generation for which a light source witha main wavelength of approximately 13.5 nm is required. In addition tothe requirement in regard to the wavelength, there are strictrequirements with respect to the light source format of thelight-emitting area and with respect to the total output which must beprovided by the source. In both respects, the known method is limited inits output. Firstly, because the geometry of the electrodes enables onlya limited access of the light-emitting area and the rest of the light iswasted on the surrounding walls. Secondly, an axial-symmetrical geometryalways forms a stretched plasma which cannot be effectivelyconcentrated. Typical lengths are currently 3 to 10 mm while thefocusing optics can process only light source formats of approximately 2mm and below.

The method can be modified in such a way that one of the electrodes isconfigured as a hollow cathode in which and/or in front of which,relative to its environment, an overpressure of the gas fill isgenerated. With a hollow cathode, the formation of the electrical fieldthat is formed in the area of the electrode recesses between theelectrodes can be influenced. By means of the bore hole area of thehollow cathode the field lines can be configured to be sufficientlystretched in order to fulfill the firing conditions for a predeterminedvoltage so that the system operates in the area of the left branch ofthe Paschen curve. Since the generated electrical discharge depends, inaddition to its dependency on the electrode spacing and on the shape ofthe electrodes, also on the gas pressure of the gas fill, it isadvantageous to generate in front of the electrode an overpressure ofthe gas fill relative to its environment. The overpressure causes thelong field lines to extend in the areas of reduced gas pressure so thathigher field strengths for the electrical firing results. As a result ofthis, the plasma generated in the case of a firing will be displacedbecause of the pressure gradient. The displacement can be realized in anarea of improved accessibility with reduced shading.

It is preferred that the gas of the gas fill is introduced via thehollow cathode whose electrode recess is used as a starting point forbuilding up a pressure drop. The starting point of the pressure drop andthus of the desired pressure gradient is thus the area of the electroderecess of the hollow cathode that is neighboring the anode.Correspondingly, a displacement of the plasma away from the electroderecess of the hollow cathode takes place.

A further improvement, in particular, of the afore described embodiment,can be achieved in that a nozzle is employed with which gas of the gasfill is blown in at high speed into the discharge vessel causingdisplacement of the plasma. In this configuration, an additional controlparameter is provided which enables shaping of the isobaric lines infront of the electrode recess of the cathode. In particular, it ispossible to move the pinch area of the plasma still father outwardly;this has advantages with regard to cooling of the device with which themethod is being carried out, in particular, in the area of theelectrodes.

The method can be improved in that, in addition to the gas that formsthe plasma, a process-affecting filling gas is introduced into thedischarge vessel. With the filling gas not only a gradient formationwith regard to the gas fill in the discharge vessel can be obtained butalso further process effects are possible. For example, the reabsorptionof EUV radiation by means of the primary gas used for the gas dischargecan be minimized. This problem is particularly dramatic when xenon isused as a discharge gas because xenon reabsorbs EUV radiationparticularly strongly. A further advantage can be that the filling gasis used in order to extinguish the discharge faster than the dischargegas in order to achieve a higher repetition rate.

Particularly advantageously the method is carried out such that thefilling gas flows into the discharge vessel in a tubular shapesurrounding the gas that forms the plasma. With the aid of the fillinggas, a very effective encompassing shaping of the discharge gas can beachieved in this way.

The invention also relates to a device having the features of thepreamble of claim 7. The device has the same disadvantages as describedabove with respect to the method so that for the device the same objectapplies as described above in connection with the method. This object issolved by the characterizing features of claim 1.

The formation of a higher gas pressure near an electrode embodied as acathode relative to an area remote from this electrode results in acorresponding pressure gradient and, in particular, in a pressure drop.A result of this pressure drop is a displacement of the plasma that isbeing formed in the sense of an excellent accessibility and reducedoptical shading with regard to an optical device for processing thelight.

One embodiment of the afore described device is expediently designedsuch that the cathode is embodied as a hollow cathode through which thegas of the gas fill can be introduced into the discharge vessel. Thehollow cathode has the stretched field lines described above whoseformation is a prerequisite in order to arrive at suitable conditionsfor the left branch of the Paschen curve. At the same time, by means ofthe hollow cathode, the gas of the gas fill, i.e., the discharge gas orprimary gas, is introduced into the discharge vessel. This provides asimple constructive configuration because a particular configuration ofspaces acting as a gas supply line is not required in this situation ofusing the hollow space of the hollow cathode for the purpose of gasintroduction.

The nozzle can be used in different ways. Advantageously, the device isconfigured such that the electrode recess and/or the central bore areconfigured as a nozzle and/or in that, by means of the nozzle, a gasflow can be generated that is oriented toward the hollow cathode. Theafore described embodiments can also be used in particular incombination with one another.

A further embodiment of the device can be realized in that the electroderecess of the cathode has a nozzle that affects the feed velocity of thegas forming the plasma and/or the gas distribution. The nozzle can beconfigured such that a significant displacement of the plasma into anarea of excellent optical accessibility is enabled.

Moreover, it may be advantageous when the cathode is surrounded at aspacing by the electrode forming the anode such that an annular space isformed and that the electrode recess of the anode is configured to openconically. In this situation, a concentric electrode arrangement isenabled which is characterized by a special freedom with regard to theaccessibility of the space into which the plasma is to be displaced. Theoptical accessibility can be further improved; the conical opening ofthe electrode recess of the anode contributes particularly to thiseffect. Even for greater observation angles relative to the commonelectrode axis, a reduced shading results, and, in the case of a shortplasma, it appears to be approximated more closely to the ideal of apoint source even at greater observation angles.

The device can be configured such that by means of the annular spacepresent between the cathode and the anode a filling gas can beintroduced into the discharge vessel. The filling gas can affect thepressure generation of the discharge gas and therefore can contribute tothe displacement and shaping of the plasma. The annular space presentbetween the cathode and the anode results in a corresponding symmetricalconfiguration of the area of the discharge vessel that is supplied withthe filling gas. In the case of rotational symmetry of the electrodes,this filling gas area is accordingly of rotational symmetry.

When the filling gas is a gas that reabsorbs extreme ultravioletradiation and/or a gas that extinguishes a plasma, reabsorption ofextreme ultraviolet radiation and/or the repetition frequency can beaffected. In pulse operation a faster course of the repeating dischargeprocesses is possible, this results in an improved light efficiency.

Minimization of the consumption of discharge gas results when the deviceis configured such that the discharge vessel is filled outwardly in thearea of the electrodes mainly with the filling gas.

Moreover, it can be advantageous to configure the device such that theaspect ratio of diameter to depth of the recess of the cathode issmaller than one. In this way, not only the gas consumption is minimizedand the gas flow is aligned so that a correspondingly large displacementof the plasma in the flow direction of the discharge gas results, butalso a contribution is provided in that current transport across thewall of the recess of the cathode and the wall of the hollow cathode andthus a weakening of the plasma is suppressed as much as possible.

The invention will be explained with the aid of the drawing. It is shownin:

FIG. 1 a diagram of the dependency of the ignition voltage from theproduct of gas pressure and electrode spacing;

FIG. 2 in a schematic illustration a first electrode arrangement; and

FIG. 3 in a schematic illustration the electrode arrangement of FIG. 2in a different operating mode.

FIG. 1 illustrates Paschen's Law, i.e., a dependency of the ignitionvoltage as a determining factor for gas discharges U₀ from the productof gas pressure p and electrode spacing d. The voltage U₀ is thatvoltage where automatically gas discharge occurs for a gas dischargepath formed between two electrodes. The law applies for a certainelectrode geometry and a certain gas. FIG. 1 clarifies that theradiation-generating method is to be carried out according to the leftbranch of the Paschen curve, i.e., with a gas discharge where thegeneration of the plasma is realized in several steps by means ofsecondary ionization processes by automatic firing and where the plasmadistribution is highly cylindrically symmetrical already in the startingphase. Energy can be introduced into the plasma, i.e., by means of apulsed current that is provided by a current source. By a suitableselection of the amplitude and of the period duration of the currentpulses, the suitable temperature of the plasma for light emission can beadjusted. The period durations are within the two-digit or three-digitnanosecond range. During one pulse, the plasma, as a result of theLorentz force, is constricted, and this results in a so-called pinch.

FIGS. 2, 3 show pinch arrangements of schematically illustratedelectrodes. The electrodes are configured to have rotational symmetryrelative to the axis of symmetry 17 . The arrangement of the electrodesis concentric. This axis of symmetry 17 is at the same time a centeraxis of an electrode embodied as a hollow cathode 14. The hollow cathode14 has a central bore 18 with an electrode recess 13 within a mouth area19 of the central bore 18, wherein this mouth area 19 is a component ofan electrode recess 20 of an additional electrode 15 embodied as ananode. The anode 15 is also of rotational symmetry and surrounds thehollow cathode 14 so that an annular space 16 is formed. Both electrodesare arranged in a discharge vessel 11 that is filled with discharge gaswhose pressure is less than that of atmospheric pressure.

A characteristic feature of the electrode recess is the configuration ofthe hollow cathode 14 that has a cavity 20 in the vicinity of theelectrode recess 13 that significantly widens the diameter of thecentral bore 18. In this way, a special configuration of the field lines21 is achieved such that, for example, the field lines 21′ extend intothe hollow space 20 and thus generate a field that extends in goodapproximation parallel to the axis of symmetry 17. When the voltage isincreased sufficiently, upon reaching the ignition voltage U₀ either anautomatic firing occurs that results in the formation of a plasma or,shortly beforehand, a triggered gas discharge is generated. The gasdischarge is generated in the vicinity of the hollow cathode 14 in frontof its end face 14′ or in front of the electrode recess 13 because herethe concentration of the electrical field is greatest and the fieldstrength decreases toward the anode 15 since it has a conically openingelectrode recess 12 where the recess wall 12′ together with the axis ofsymmetry 17 forms an acute angle up to a 90 degree angle. The conicalrecess wall 12′ of the electrode recess 12 is arranged relative to thehollow cathode 14 such that the smallest recess diameter of the anode15, which is identical to the outer diameter of the annular space 16provided between the electrodes, is arranged at the level of the endwall 14′ of the hollow cathode 14.

The central bore 18 is formed as a gas inlet 22. Via the gas inlet 22the discharge gas is introduced through the central bore 18 into thecavity 20 and can flow from here through the electrode recess 13 of thehollow cathode 14 into the electrode recess 12 of the anode and into thedischarge vessel where vacuum is maintained. The pressure of thedischarge gas can drop already within the hollow cathode 14. In anycase, starting at the electrode recess 13 a pressure drop can begenerated. FIG. 2 shows isobaric lines 23 of decreasing pressure. As aresult of the resulting pressure gradient of the gas fill, the plasma 10is displaced in the direction away from the cathode. The displacement isrealized as a result of the symmetrical configuration of the electricalfield and the gas pressure distribution in the direction of the axis ofsymmetry 17.

The amount of displacement of the plasma 10 depends on the dimensions ofthe electrode recess 13 and of the flow velocity of the gas. Forexample, the electrode recess 13 can be configured as a nozzle withwhich the gas of the gas fill is blown at high speed into the dischargevessel 11. The arrangement of the plasma 10 in the electrode recess 12can be changed largely in that suitable electrical and aerodynamicconditions are selected. In particular, in the described configurationof the anodes it can be achieved that the plasma 10 no longer has acylindrical geometry but, corresponding to the illustration, isconcentrated in a reduced, more egg-shaped volume. Simultaneously withthe displacement, a deformation of the plasma 10 takes place that isadvantageous with regard to optical considerations.

The above discussions also hold true for the configuration of FIG. 3.Here, the special feature is illustrated that the annular space 16provided between the electrodes is used as a gas inlet 24. For example,a filling gas 25 is used that flows, as a result of the annular ortubular configuration of the annular space 16, in a correspondinglytubularly shaped flow surrounding the gas that forms plasma 10 into thedischarge vessel 11. This filling gas 25 has a shaping effect on the gasforming the plasma. FIG. 3 shows in comparison to FIG. 2 theconstricting effect of the filling gas 25, shown in dash-dotted lines,onto the gas forming the plasma and thus onto its isobaric lines 23.Also, with the aid of the filling gas it is thus possible to createpressure changes within the plasma-generating gas which, in turn, causedisplacements and/or shaping of the plasma 10.

FIG. 3 shows a nozzle 26 with which gas supply of a discharge gas to thefront sides of the electrodes can be realized. The exit velocity of thegas must be high enough in order to generate, according to arrow 27, agas flow oriented toward the hollow cathode 14. By means of the gasflow, in front of the end face 14′ of the hollow cathode 14 a higherpressure is generated which drops greatly corresponding to the isobariclines 23 relative to the background.

The afore described configuration of the electrodes for pinch shapingvia inhomogeneous pressure conditions of the gas contained within thedischarge vessel 11, in particular, of the gas present in front of theend face 14′ of the hollow cathode, can be supplemented by means ofpredetermined sizing of the electrode recess 13. In particular, it isadvantageous to configure the electrode recess 13 as a cathode openingsuch that the aspect ratio of diameter d to depth b is <1. This resultsnot only in a more uniform flow of the gas when it is supplied throughthe electrode recess 13 into the discharge vessel 11, but also allowsaffecting the transport of the charge carriers for the plasma. Inparticular, the current transport via the electrode recess 13 and viathe wall of the hollow cathode forming the cavity 20 is suppressed asmuch as possible. This also improves the generation of the plasma 10 inan area of the electrode recess 12 where an excellent opticalaccessibility is provided, i.e., without shading of the plasma even atgreater observation angles relative to the axis of symmetry 18.

1-15. (canceled).
 16. A method for generating a radiation of extremeultraviolet range or soft x-ray range by gas discharge, the methodcomprising the steps of: arranging two electrodes having an electroderecess, respectively, in a discharge vessel, wherein the electroderecesses are coaxial to one another; connecting the two electrodes tohigh voltage; providing a gas fill between the electrodes in an area ofthe electrode recesses, wherein the gas fill has a predetermined gaspressure in accordance with a discharge operation realized on a leftbranch of the Paschen curve; generating a plasma emitting the radiationwhen supplying high voltage; displacing or deforming the plasma byproviding a pressure gradient of the gas fill in the area of theelectrode recesses.
 17. The method according to claim 16, furthercomprising the step of configuring a first one of the two electrodes asa hollow cathode and generating in the hollow cathode or in front of thehollow cathode an overpressure of the gas fill relative to anenvironment of the hollow cathode.
 18. The method according to claim 17,further comprising the step of introducing the gas of the gas fill viathe hollow cathode and generating a pressure drop of the gas fillbeginning at the electrode recess of the hollow cathode.
 19. The methodaccording to claim 18, further comprising the step of arranging a nozzlein the discharge vessel and blowing the gas of the gas fill at highspeed into the discharge vessel and causing a displacement of theplasma.
 20. The method according to claim 16, further comprising thestep of introducing, in addition to the gas fill, a process-affectingfilling gas into the discharge vessel.
 21. The method according to claim20, wherein, in the step of introducing the filling gas, the filling gasflows in a tubular shape surrounding the gas of the gas fill forming theplasma into the discharge vessel.
 22. A device for generating aradiation of extreme ultraviolet range or soft x-ray range by gasdischarge, the device comprising: a discharge vessel; two electrodesconnected to high voltage and arranged within the discharge vessel; thetwo electrodes having an electrode recess, respectively, wherein theelectrode recesses are coaxial to one another; a gas fill arrangedbetween the electrode recesses having a predetermined gas pressure inaccordance with discharge operation on the left branch of the Paschencurve, wherein a plasma emitting the radiation is generated in the gasfill when high voltage is supplied; wherein the gas pressure of the gasfill is higher near a first one of the two electrodes configured as acathode than in an area of the discharge vessel remote from the cathode.23. The device according to claim 22, wherein the cathode is a hollowcathode through which the gas of the gas fill is introduced into thedischarge vessel.
 24. The device according to claim 23, wherein thecathode has a first nozzle.
 25. The device according to claim 24,wherein the first nozzle is configured to increase a supply speed of thegas of the gas fill.
 26. The device according to claim 25, wherein thefirst nozzle is configured to affect a distribution of the gas of thegas fill.
 27. The device according to claim 24, wherein the first nozzleis configured to affect a distribution of the gas of the gas fill. 28.The device according to claim 24, wherein at least one of the electroderecess and a central bore of the cathode forms the nozzle.
 29. Thedevice according to claim 24, further comprising a second nozzlearranged in the discharge vessel and configured to guide a gas flowtoward the cathode.
 30. The device according to claim 22, furthercomprising a nozzle arranged in the discharge vessel and configured toguide a gas flow toward the cathode.
 31. The device according to claim22, wherein a second one of the electrodes is an anode and wherein theanode surrounds the cathode at a spacing so as to form an annular spacebetween the cathode and the anode, wherein the electrode recess of theanode opens conically.
 32. The device according to claim 31, wherein afilling gas is introduced into the discharge vessel via the annularspace.
 33. The device according to claim 32, wherein the filling gas isa gas reabsorbing extreme ultraviolet radiation or a gas extinguishingthe plasma.
 34. The device according to claim 32, wherein the dischargevessel is filled outwardly in the area of the electrodes mainly with thefilling gas.
 35. The device according to claim 22, wherein an aspectratio of a diameter of the electrode recess of the cathode relative to adepth of the electrode recess of the cathode is smaller than one.